Low Z Impurity Seeding in KSTAR H-mode Plasmas*

Low Z impurity seeding experiments using nitrogen and neon were recently carried out in KSTAR. The discharges were in H-mode with a total heating power of 3.5 MW. Maximum achieved total radiation fractions (f_rad,tot = P_rad,tot/P_heat) were about 80% for N and 50% for Ne by the impurity seeding rate ramp-up. N showed a larger radiated power fraction in the divertor region (= P_rad,div/P_rad,tot) compared to Ne with the same f_rad,tot, and also showed a larger reduction in the particle and heat fluxes onto the outer target than Ne. The inner target particle flux remained nearly constant and this in/out detachment asymmetry opposite to most other devices was also reported in KSTAR L-mode discharges. An experiment at higher heating power (6.5 MW) was performed in the 2022 campaign and the outer target particle flux reduction was obtained by Ne seeding. Numerical simulations using the SOLPS-ITER code of the moderate power discharges were performed. The simulations qualitatively reproduced the experimentally observed radiation behaviors of N and Ne, although both species dissipated the particle flux onto the outer target efficiently (by a factor of 2). Analysis of impurity transport in the divertor will be presented to provide an explanation of the different behaviors between N and Ne.